11The aluminum magnesium alloy system has notable attributes, such as a high strength to weight 12 ratio and weldability, but it can become susceptible to sensitization at relatively low 13 temperatures, which can lead to stress corrosion cracking. It is well established that growth of the 14 secondary β phase is temperature driven, however there is little understanding about the role of 15 prior strain in the β phase nucleation and growth process. Understanding the effect of cold rolling 16 will also bring new insight into better themomechanical treatments leading to a more appropriate 17 temper to improve sensitization resistance. In this study cold rolled AA5456-H116 is observed 18 during in situ transmission electron microscopy heating experiments. The results of this study 19 show the impact of an increased dislocation density, due to cold rolling, on β phase precipitation 20 as well as the effect of misorientation on growth and kinetics. The effect of additional 21 dislocations is also observed to show an increase in precipitate density and a lowering of 22 nucleation temperature attributed to Mg pipe diffusion. 23 24 29 marine, automotive, and aircraft industries as well as food handling and chemical industries [1-30 4]. As global warming plays a larger part in material selection, further development on Al-Mg 31 usage has been conducted to improve fuel consumption and emissions, as well as increase 32 recycling potential of final automotive components [1][2][3][4]. Use of this material in service, 33 however, has been undermined by stress corrosion cracking [5] resulting from sensitization at 34 moderate and even low temperatures [6]. Growth of a deleterious secondary phase, known as the 35 β phase (Al 3 Mg 2 ), during sensitization of Al-Mg alloys has been a problem for in-service naval 36 vessels for many years [7]. Specifically, while exposed to harsh in-service environments, such as 37 sea water, a galvanic couple is formed between the aluminum matrix and the β phase 38 precipitates[8].This ultimately leads to a preferential dissolving of these precipitates that result in 39 intergranular cracking [4,[8][9][10]. But it is possible for crack propagation to occur from non-40 continuous beta precipitates as well [11]. 42It is well known that the formation of β phase is temperature driven [12][13][14] but the nucleation 43 and growth mechanisms are not fully understood. It is generally agreed upon that during 44 exposure to elevated temperatures (50-300°C) the Mg segregation begins from the supersaturated 45 solution (>3%Mg or higher[1]),resulting in the formation of a final, stable β equilibrium phase[5, 46 differential calorimetric analysis, indicating that a direct transformation from its metastable 48 precipitate form, β', was not required. The majority of studies [16][17][18] have shown β phase 49 precipitation during long term growth periods at elevated temperatures and have observed the 50 formation of this secondary phase occurs at the grain boundaries (GBs), sub-grains, and defects 51...

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Recently, high entropy alloys (HEAs) and in particular Al x CoCrFeNi have received considerable attention due to its promising thermal, mechanical, and corrosion properties. This unconventional alloy design contains at least five principal elements with atomic concentrations between 5 and 35 atomic percent [1]. Instead of forming intermetallics, these alloys tend to crystallize into single phase solid solutions, and most have either face-centered cubic (FCC) or body-centered cubic (BCC) structures, due to high configurational entropy, sluggish diffusion, and severe lattice distortion [2]. Formation of secondary phases are reported in recent studies at intermediate and high annealing temperatures of the Al x CoCrFeNi HEA, however, many of these studies focus on late stages of precipitation [3][4][5][6]. The present work is motivated by the need for a better understanding and controlling the phase stability of as-homogenized Al x CoCrFeNi (x=0.0, 0.1, 0.3, 0.5) HEA at intermediate and high annealing temperatures.In this study, we employ the in-situ TEM heating technique to track the early, intermediate and final stages of intermediate phases precipitation in Al x CoCrFeNi HEA. As-homogenized HEAs were in-situ heat treated in a JEOL 2100 LaB 6 TEM, equipped with a high-resolution pole piece, using a Gatan heating holder at ramp rate of 0.5 °/s. Scanning transmission electron microscope(STEM)-equipped with energy dispersive x-ray spectroscopy (EDS) was used to study the chemical composition of the precipitates. Following STEM-EDS, selected area diffraction patterns(SAED) technique was performed to examine the crystal structure of as-homogenized and annealed samples. We find that NiAl, Co-rich and Cr-rich precipitates formed in this alloy after undergoing in-situ TEM heating at 550 °C, 700 °C, and 900 °C for different annealing periods. The study was coupled with phase and orientation analysis using precession electron diffraction techniques to examine the effect of grain boundary character on the precipitation of second phases. Figure 1(a-e) shows the representative high-angle annular dark-field (HAADF)-(STEM), and EDS analysis of in-situ TEM annealed CoCrFeNi HEAs at 700°C for 10 min. Overall, the work described provides a foundation for understanding the stability window for candidate HEAs in extreme environments. These results are discussed in the context of the growing literature comparing the ideal methods for stabilizing mechanisms in HEAs for use in high temperature environments.

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.